A Master Mariner or MM is the professional qualification required for someone to serve as the person in charge or person in command of a commercial vessel. In England,[where?] the term Master Mariner has been in use at least since the 13th century, reflecting the fact that in guild or livery company terms, such a person was a master craftsman in this specific profession (e.g. Master Carpenter, Master Blacksmith, etc. [2]). In the United States, the term was introduced in the mid-19th century.

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In the UK, the term “Master Mariner” is reserved only for those who hold an unlimited master's certificate. The term “unlimited” (often referred to as unrestricted) indicates that there are no limits on size, power or geographic location on the certificate. It is the highest level of professional qualification amongst mariners and Deck Officers in the British Merchant Navy. The Extra Master's qualification, which was discontinued in the 1990s, used to be the highest professional qualification and it was the pinnacle for any mariner to achieve. There are also various other levels of Master's certificates, which may be restricted or limited to Home Trade/Near Coastal Voyages and/or by gross tonnage. The holder of a restricted Master's certificate is not referred to as a "Master Mariner".

In the British Merchant Navy a Master Mariner who has sailed in command of an oceangoing merchant ship will be titled Captain.[citation needed] A professional seafarer who holds a restricted or limited Master's certificate who has sailed in command of a ship (i.e. appropriate to the size, power or geographic limits of their certificate) can also be titled Captain.

An unrestricted master's certificate is colloquially called a "Master's Ticket" or a "Master's." It is sometimes still referred to as a “Class 1” or “Master Foreign-Going” as it was named during the latter part of the 20th century. The conventions or acts governing the certificate have evolved alongside the shipping industry and the official name of a Master Mariner’s qualification has varied over the years. The UK Maritime & Coastguard Agency, in-line with the amended STCW convention,[1] currently[when?] title the certificate Master Unlimited.[2]

A Master Mariner license comes rated for the maximum gross tonnage of vessel that license-holder is allowed to operate with the highest being a license for "any gross tonnage", colloquially referred to as "unlimited". Additionally, the licenses come with restrictions as to what waters the officer is allowed to operate on (inland, coastal, oceans). An unlimited tonnage ocean license is the highest level license attainable.[3] A Master Mariner who has sailed in command of a commercial vessel is given the title and the appellative of Captain. Also, in common usage anyone who has achieved any USCG master's license commonly refers to themselves as "Captain", regardless of limitations on that license or lack of commercial or command experience.